Ferritic heat-resisting steel

ABSTRACT

A ferritic heat-resisting steel that shows a slight decrease in creep strength at the heat affected zone of the welded joint. The steel is characterized by consisting of, by mass %, C: less than 0.05%, Si: not more than 1.0%, Mn: not more than 2.0%, P: not more than 0.030%, S: not more than 0.015%, Cr: 7-14%, V: 0.05-0.40%, Nb: 0.01-0.10%, N: not less than 0.001% but less than 0.050%, sol. Al: not more than 0.010%, and O (oxygen): not more than 0.010%, with the balance being Fe and impurities, and further characterized in that the density of carbide and carbonitride precipitates contained with a grain diameter of not less than 0.3 μm is not more than 1×10 6 /mm 2 . This steel may further contain one or more of the following elements: a total of 0.1-5.0% of Mo and W; a total of 0.02-5.00% of Cu, Ni and Co; a total of 0.01-0.20 of Ta, Hf, Nd and Ti; a total of 0.0005-0.0100% of Ca and Mg; and 0.0005-0.0100% of B.

TECHNICAL FIELD

[0001] This invention relates to a ferritic heat-resisting steel showinga low level of softening in the welding heat affected zone.

BACKGROUND ART

[0002] Among high-temperature materials for use in heat and pressureresisting piping systems in boilers, chemical plants and so forth, thereare low Cr ferritic steels, typically 2·¼Cr-1Mo steel, high Cr ferriticsteels, typically 9Cr-1Mo steel, and austenitic stainless steels,typically 18Cr-8Ni steel.

[0003] Among them, high Cr ferritic steels are superior to low Crferritic steels in strength and corrosion resistance in the temperaturerange of 500-60° C. High Cr ferritic steels are also superior toaustenitic stainless steels in price and stress corrosion crackingresistance. Furthermore, high Cr ferritic steels have a low coefficientof thermal expansion and show smaller strains in response to temperaturechanges. Thus, high Cr ferritic steels, which have many advantages asmaterials for use at high temperatures, are currently in wide use.

[0004] In recent years, the environment for use thereof has becomeincreasingly severe, and accordingly, the use performance requirementsthat are imposed on heat-resisting ferritic steels, in particular thecreep strength requirement, have become much larger. Therefore, a numberof improvements have been proposed. Those are new heat-resistingferritic steels based on ferritic steels containing 8-13% of Cr andimproved in strength at elevated temperatures by adjusting the contentof Mo, W, Nb, V as well as Co, Ta, Nd, Zr, B and so forth, and a numberof methods for heat treatment thereof (cf. e.g. Japanese laid-openpatent specifications (JP Kokai) Nos. H02-310340, H04-6213, H04-350118,H04-354856, H05-263196 and H05-311342 to 311346).

[0005] It is known that when heat-resisting ferritic steels are used inwelded structures, the creep strength of welded joints declines by 20%or more in the heat affected zone (HAZ). The phenomenon is called “HAZsoftening”, which is described for example in “Science and Technology ofWelding and Joining, 1996, Vol. 1, No. 1, pp. 36-42”.

[0006] However, as for the ferritic steels and the methods of productionthereof as disclosed in the above-cited publications, the main objectiveis to improve the creep strength and/or toughness of the base metals. Noattention has been paid at all to the decreases in creep strength ofwelded joints as a result of the HAZ softening phenomenon.

[0007] For suppressing the HAZ softening phenomenon, a number ofheat-resisting ferritic steels and methods of production thereof havealso been proposed (cf e.g. JP Kokai H05-43986, H06-65689, H07-242935,H08-85848, H08-337813, H09-13150, H09-71845 and H111-106860).

[0008] However, the ferritic steels and methods of production asdisclosed in those publications require a special melting and/orthermo-mechanical treatment, as shown in JP Kokai H07-242935 or JP KokaiH08-337813 for example and therefore problems arise such as an increasein production cost and/or a decrease in production efficiency. Steelsdisclosed in JP Kokai H06-65689, H08-85848 and H09-71845 contain Taoxide particles and such expensive elements as Ta, Nd and/or Hf asessential components and therefore there is the problem of an increasein production cost.

DISCLOSURE OF THE INVENTION

[0009] An objective of the present invention is to provide a ferriticheat-resisting steel that is inexpensive and shows only a slightdecrease in creep strength in the heat affected zone of welded joints.The steel requires no particular melting or thermo-mechanical treatmentand does not always require the addition of expensive Ta oxideparticles, Ta, Nd, Hf and the like.

[0010] The ferritic heat-resisting steel of the invention ischaracterized by the following features (A) and (B):

[0011] (A) The chemical composition consists of, by mass %, C: less than0.05%, Si: not more than 1.0%, Mn: not more than 2.0%, P: not more than0.030%, S: not more than 0.015%, Cr: 7-14%, V: 0.05-0.40%, Nb:0.01-0.10%, N: not less than 0.001% but less than 0.050%, sol. Al: notmore than 0.010%, and O (oxygen): not more than 0.010%, with the balancebeing Fe and impurities.

[0012] (B) The density of carbide and carbonitride precipitates,contained in the steel and having a diameter of not less than 0.3 μm, isnot more than 1×10⁶/mm².

[0013] The ferritic heat-resisting steel of the invention may contain atleast one component selected from one or more groups given below, inlieu of part of Fe in the composition (A) mentioned above.

[0014] First group: a total content of 0.1-5.0 mass % of Mo and W.

[0015] Second group: a total content of 0.02-5.00 mass % of Cu, Ni andCo.

[0016] Third group: a total content of 0.1-0.20 mass % of Ta, Hf, Nd andTi.

[0017] Fourth group: a total content of 0.0005-0.0100 mass % of Ca andMg.

[0018] Fifth group: 0.0005-0.0100 mass % of B.

[0019] The inventors paid attention to micro-structural changes due tothermal cycles in welding and carried out repeated experments andinvestigations. As a result, they obtained the following new findingsand have now completed the present invention.

[0020] First, it was revealed that HAZ softening occurs according to thefollowing mechanisms. In the production of base metals, M₂₃C₆ typecarbides (in this case, M being such a metal element as Cr, Mo or W) orMX type carbonitrides (in this case, M being such a metal element as Vor Nb, and X representing C and N) precipitate. Among them the M₂₃C₆type carbides, containing a large amount of Cr as a solid solution, arecoarse as compared with the MX type carbonitrides, and they are partlydecomposed by thermal cycles at the welding stage and dissolved andcontained as a solid solution in the matrix. During the subsequent heattreatment (post-welding heat treatment) and in the earlier stage ofcreep, the Cr contained as a solid solution in a supersaturatedcondition again finely precipitates from the matrix regions, whereinsaid part of M₂₃C₆ type carbides have become solid solution. Therefore,compared with the base metal (where the partial dissolution of carbidesas solid solution does not occur) which is not subjected to weldingthermal cycles or the part where the HAZ softening does not occur (wherethe partial dissolution of carbides as solid solution does not occur, orthe carbides are completely decomposed and dissolved as solid solution),the density and size of M₂₃C₆ type carbide precipitates which contain Cras a main component become uneven or irregular in the HAZ. During use,the precipitation of the above-mentioned Cr solid-soluted in asupersaturated condition becomes complete, and after arrival of the Crconcentration in the base metal at an equilibrium concentration, theparticles become coarse due to the disappearance of finer particles.Thus, Cr-based fine M₂₃C₆ type carbides disappear, and the Cr is fed tothe surrounding M₂₃C₆ type carbides to promote the growth thereof, orthe Cr re-precipitates and grows utilizing MX type carbonitrides asnuclei. The rate of growth of M₂₃C₆ type carbides and MX typecarbonitrides increase. As a result, the effect of dispersionstrengthening by fine MX type carbonitrides, which greatly contribute tostrengthening, is impaired at an early stage, whereupon the strengthdecreases.

[0021] Based on the above findings, the inventors made detailedinvestigations in search of a method of preventing the HAZ softening,and as a result, it was confirmed that the following measures areeffective in preventing the HAZ softening.

[0022] (a) Reducing the amount of coarse precipitates (mainlyCr-containing M₂₃C₆ type carbides) existing in the steel before welding,and thereby increasing uniformity in the size of precipitates asresulting from said partial solid solution due to welding thermalcycles.

[0023] (b) For reducing the amount of coarse M₂₃C₆ type carbideprecipitates, it is very effective to reduce the contents of C and N,which lower the activity of Cr.

[0024] (c) Reductions in C and N content are effective in increasing theequilibrium Cr concentration in the base metal, and retarding the rateof growth of precipitates (M₂₈C₆ type carbides and MX typecarbonitrides) in the process of coarsening thereof, after completion ofthe precipitation of M₂₃C₆ type carbides and arrival of the Crconcentration in the base metal at an equilibrium concentration duringuse.

[0025] More specifically, it was confirmed that the decrease in strengthin the HAZ can be prevented by reducing the density of M₂₃C₆ typecarbide and MX type carbonitride precipitates with a diameter (majoraxis) of not less than 0.3 μm to not more than 1×10⁶/mm², and byreducing the content of C and N respectively to a level lower than0.05%.

[0026] The above findings (a), (b) and (c) are quite different from thetechnical idea in the inventions disclosed in the above-cited JP KokaiH05-43986 and H08-85848 wherein intentional addition of C and N isnecessary to secure creep strength. The above findings also differ fromthe technical idea in the invention disclosed in JP Kokai H07-242935wherein causing a large amount of fine M₂₃C₆ type carbides (specificallyCr₂₃C₆) to precipitate is necessary.

BEST MODES FOR CARRYING OUT THE INVENTION

[0027] The ferritic heat-resisting steel of the invention ischaracterized in that it satisfies the above-mentioned conditions (A)and (B). The grounds for specifying the chemical composition and thesize and precipitation density of M₂₃C₆ type carbides and MX typecarbonitrides are as follows. In the following description, means “% bymass”.

[0028] I. Chemical Composition

[0029] C: less than 0.05%

[0030] C has been regarded as an element forming M₂₃C₆ type carbides andcontributing to improved strength at elevated temperatures. However, asmentioned above, some M₂₃C₆ type carbides become solid solution uponwelding and reprecipitate as coarse M₂₃C₆ type carbides during thesubsequent heat treatment and in the earlier stage of creep process,causing irregularity in size and the HAZ softening. Therefore, forreducing the amount of M₂₃C₆ type carbide precipitates before weldingand providing the long-term strength of the HAZ, namely for preventingthe HAZ softening, it is effective to reduce the C amount as much aspossible. Thus, the C amount should be less than 0.05%, and preferablynot more than 0.045%. The lower limit is not particularly prescribed.However, C is an element effective in forming fine MX typecarbonitrides, which have a dispersion strengthening effect, and such aneffect can be obtained when its content is not less than 0.001%.Therefore, not less than 0.001% of C may be contained in the steel whenthis effect is desired.

[0031] Si: not more than 1.0%

[0032] Si is added as a deoxidizer at the steel making stage. Si is alsoan element that improves the oxidation resistance and high-temperaturecorrosion resistance. However, excessive addition causes creepembrittlement and a decrease in toughness. Therefore, the Si amountshould be not more than 1.0%, and preferably not more than 0.8%. Incases where deoxidation is realized to a sufficient extent by Mn and/orAl to be mentioned later, no intentional addition of Si is necessary,hence the lower limit of the amount of Si is not prescribed inparticular. However, for ensuring the deoxidizing effect with Si, it isdesirable that the Si amount be not less than 0.03%.

[0033] Mn: not more than 2.0%

[0034] Like the above-mentioned Si, Mn is added as a deoxidizer at thesteel-making stage. Mn is an austenite-forming element and is alsoeffective in obtaining a martensitic structure. However, an excessiveamount thereof causes creep embrittlement and decreases in creepstrength. Therefore, the Mn amount should be not more than 2.0%, andpreferably not more than 1.8%. In cases where deoxidation is realized toa sufficient extent by the above-mentioned Si and/or Al to be mentionedlater, no intentional addition of Mn is necessary, hence the lower limitis not prescribed in particular. However, for ensuring the deoxidizingeffect with Mn, it is desirable that the Mn amount be not less than0.03%.

[0035] P: not more than 0.30%

[0036] P is an impurity contained in the steel. When its content isexcessive, it causes grain boundary embrittlement. Therefore, the upperlimit thereof should be 0.030%. The P amount should be as low aspossible.

[0037] S: not more than 0.15%

[0038] Like P mentioned above, S is an impurity element contained in thesteel, and when its amount is excessive it causes grain boundaryembrittlement. Therefore, the upper limit thereof should be set at0.015%. The S amount also should be as low as possible.

[0039] Cr: 7-14%

[0040] Cr is an element effective in providing oxidation resistance athigh temperatures, high-temperature corrosion resistance and strength atelevated temperatures. For obtaining these effects, an amount of notless than 7% is necessary. However, at excessive addition levels, itincreases the formation of Cr-based M₂₃C₆ type carbides and promotes therate of growth of carbides, causing decreases in creep strength in theHAZ. Therefore, the upper limit of the Cr amount should be 14%. A Cramount of 8-13% is more preferable.

[0041] V: 0.05-0.40%

[0042] V is an element that forms fine MX type carbonitrides, which arestable even at elevated temperatures, and contributes to the improvementof creep strength. For obtaining this effect, an amount of not less than0.05% is necessary. However, when its amount exceeds 0.40%, it causescoarsening of MX type carbonitrides and the strength improving effectowing to fine dispersion thereof is lost at an early stage, and inaddition, it causes a decrease in toughness. Therefore, the upper limitof the V content should be 0.40%, and 0.10-0.30% is more preferable.

[0043] Nb: 0.01-0.10%

[0044] Nb, like the above-mentioned V, forms fine MX type carbonitrides,which are stable even at elevated temperatures, and contributes to theimprovement of creep strength. An amount of not less than 0.01% isnecessary in order to obtain this effect. However, when its amountexceeds 0.10%, it causes coarsening of MX type carbonitrides and thestrength improving effect owing to fine dispersion thereof is lost at anearly stage, and in addition, it causes a decrease in toughness.Therefore, the upper limit of the Nb amount should be 0.10%, and0.02-0.08% is more preferable.

[0045] N: not less than 0.001% but less than 0.050%

[0046] As with C mentioned above, N is effective in reducing theactivity of Cr, and promotes the precipitation of M₂₃C₆ type carbidesand promotes the HAZ softening. Therefore, N content should be reducedas much as possible. The upper limit of the N content is less than0.050%. On the other hand, N is also an element that forms MX typecarbonitrides, in which V and Nb are contained as a solid solution, thusproducing the fine dispersion strengthening effect thereof For obtainingsuch an effect, content of not less than 0.001% is necessary. For thesereasons, N content should be not less than 0.001% but less than 0.050%.0.003-0.045% is more preferable.

[0047] sol. Al: not more than 0.010%

[0048] Al is added as a deoxidizer at the steel-making stage but anexcessive addition causes a decrease in the steel's cleanliness.Therefore, the sol.Al content should be not more than 0.010%, andpreferably not more than 0.008%. In cases where the above-mentioned Siand/or Mn realize deoxidation to a sufficient extent, no intentionaladdition of Al is necessary; hence the lower limit of the Al content isnot prescribed in particular. However, for ensuring the deoxidizingeffect with Al, it is desirable that the sol. Al content be not lessthan 0.003%.

[0049] O (oxygen): not more than 0.010%

[0050] O (oxygen) is an impurity contained in steel. When it iscontained in excess, it causes a decrease in the steel's cleanliness,and in addition causes a decrease in creep strength. Therefore, the Ocontent should be not more than 0.010%. O content should be as low aspossible.

[0051] The remaining portion, other than the above alloying elements andimpurities, is substantially accounted for by Fe. However, wherenecessary the following components may be added in lieu of part of Fe.

[0052] Mo, W:

[0053] The intentional addition of these elements is not alwaysnecessary. However, when added, both elements are effective in solidsolution hardening of the matrix, and furthermore precipitate asintermetallic compounds, contributing to an improvement in creepstrength. Therefore, when such an effect is desired, one or both may beadded intentionally and the effect becomes significant at a total amountof not less than 0.1%. However, when the total amount exceeds 5.0%, theamount of coarse intermetallic compounds increases, causing a decreasein toughness. Therefore, when these elements are added, the total amountshould be 0.1-5.0%. A preferred total amount is 0.5-4.5%.

[0054] Cu, Ni, Co:

[0055] The intentional addition of these elements is not alwaysnecessary. When added, they contribute to martensitic matrix structureformation because they are all austenite-forming elements. Therefore,when such an effect is desired, one or more of them may be addedintentionally. The effect becomes significant at a total amount of notless than 0.02%. However, when the total amount exceeds 5.00%, the creepductility is markedly reduced. Therefore, when they are added, the totalamount of these elements should be 0.02-5.00%, and preferably0.05-4.50%.

[0056] Ta, Hf, Nd, Ti:

[0057] The intentional addition of these elements is not alwaysnecessary. When added, all the elements such as the above-mentioned Vand Nb form MX type carbides and contribute to an improvement in creepstrength. Therefore, when such an effect is desired, one or more of themmay be added. The effect becomes significant at a total amount of notless than 0.01%. However, when the total amount exceeds 0.20%, thecarbides become coarse and the cleanliness of the steel deteriorates,and the toughness is impaired. Therefore, when they are added, the totalamount of these elements should be 0.01-0.20%, and preferably0.03-0.18%.

[0058] Ca, Mg:

[0059] The intentional addition of these elements is not alwaysnecessary. When added, both of these elements improve the hotworkability of the steel. Therefore, when such an effect is desired, oneor both may be added intentionally. The effect becomes significant at atotal amount of not less than 0.0005%. However, when the total amountexceeds 0.0100%, the cleanliness of the steel is impaired. Therefore,when they are added, the total amount of these elements should be0.0005-0.0100%, and preferably 0.0010-0.0080%.

[0060] B:

[0061] It is not always necessary to add B intentionally. When added, itdisperses and stabilizes carbides and contributes to the improvement increep strength of the base material. B is also an element improvinghardenability of the steel and is effective in rendering the structureof the base metal martensitic. Therefore, when these effects aredesired, it may be added intentionally. The effects become significantat a level of not less than 0.0005%. However, when the content exceeds0.0100%, the high-temperature crack resistance during welding isimpaired. Therefore, when B is added, content of 0.0005-0.0100% isrecommended, and preferably 0.0010-0.0080%.

[0062] II. Sizes and Amount of M₂₃C₆-Based Carbides and MX TypeCarbonitrides in the Steel

[0063] As mentioned hereinabove, the decrease in creep strength in theHAZ is caused by the following process; Carbides, mainly coarse M₂₃C₆type carbides, which have precipitated in the step of base metalproduction, become solid solution partly during thermal cycles in thestep of welding. Fine carbides precipitate again from the regionscontaining said partly solid-solute carbides during the subsequent heattreatment and in the early stage of creep process. Thus, the density andsizes of Cr-based carbide precipitates unevenly compared with the basemetal, which has not been subjected to welding thermal cycles or theportions showing no HAZ softening.

[0064] In order to prevent said phenomenon, it is effective to restrictthe amount of the above-mentioned carbides, mainly M₂₃C₆ type carbides,and MX type carbonitrides produced in the base metal before welding, andto reduce the amount of the carbides which solid-solute partly duringthermal cycles at the welding stage. For obtaining the effect to asatisfactory extent, it is necessary to reduce the density ofprecipitates of carbides mainly of the M₂₃C₆ type and MX typecarbonitrides not smaller than 0.3 μm in diameter (major axis), in thebase metal before welding, to a level not higher than 1×10⁶/mm². Thereasons will be shown in examples to be mentioned later.

[0065] A structure where the density of precipitates of carbides, mainlyM₂₃C₆ type carbides, and MX type carbonitrides, not smaller than 0.3 μmin diameter (major axis), is not higher than 1×10⁶/mm² can be attainedby appropriately adjusting the heat treatment temperature and thekeeping time in “normalizing” or “normalizing+tempering” during basemetal production according to the chemical composition of the steel (forexample employing the conditions shown in the examples given later).

EXAMPLES

[0066] 12-mm-thick steel plates were prepared from 34 ferritic steelswith respective having the chemical compositions shown in Table 1 andTable 2. In preparing the steel plates, the steels were melted in avacuum-melting furnace and formed into plates by casting, hot forgingand hot rolling. The plates were normalized by maintaining a temperaturerange within 900° C. to 1180° C. for 0.5 hour, and then tempered bymaintaining a temperature range within 700° C. to 770° C. for 1 to 10hours. In some examples, the tempering stage was omitted.

[0067] During the above procedure, the surface of each plate after hotrolling was examined for defects by visual observation and the hotworkability of each steel was evaluated. The hot workability wasevaluated as good “{circle over (∘)}” when the number of defects per mm²was 5 or less; no problem “◯” when the number was 6 to 20; and poor “X”when the number was 21 or more. The results are also shown in Table 2.TABLE 1 Chemical Composition (mass %, bal.: Fe and impurities) No. C SiMn P S Cr V Nb N sol. Al O Mo W Steel of This Invention 1 0.035 0.250.44 0.015 0.006 9.16 0.19 0.04 0.014 0.004 0.004 — — 2 0.045 0.30 0.610.012 0.004 9.46 0.21 0.05 0.020 0.005 0.003 — — 3 0.020 0.28 0.41 0.0120.004 9.25 0.22 0.05 0.018 0.006 0.004 — — 4 0.001 0.26 0.52 0.009 0.0059.12 0.20 0.08 0.016 0.005 0.004 — — 5 0.005 0.20 0.48 0.010 0.005 9.200.19 0.06 0.023 0.005 0.003 — — 6 0.016 0.19 0.47 0.013 0.006 9.23 0.170.06 0.028 0.004 0.004 — — 7 0.049 0.25 0.33 0.015 0.003 8.04 0.16 0.050.014 0.006 0.005 0.96 — 8 0.049 0.25 0.33 0.015 0.003 8.04 0.16 0.050.014 0.006 0.005 0.96 — 9 0.049 0.25 0.33 0.015 0.003 8.04 0.16 0.050.014 0.006 0.005 0.96 — Comparative Example 10 0.049 0.25 0.33 0.0150.003 8.04 0.16 0.05 0.014 0.006 0.005 0.96 — 11 0.049 0.25 0.33 0.0150.003 8.04 0.16 0.05 0.014 0.006 0.005 0.96 — 12 0.049 0.25 0.33 0.0150.003 8.04 0.16 0.05 0.014 0.006 0.005 0.96 — 13 0.049 0.25 0.33 0.0150.003 8.04 0.16 0.05 0.014 0.006 0.005 0.96 — Steel of This Invention 140.047 0.22 0.50 0.017 0.002 8.94 0.23 0.07 0.022 0.003 0.003 — 2.95 150.035 0.26 0.48 0.018 0.004 10.51 0.15 0.03 0.010 0.005 0.003 0.42 1.8816 0.018 0.31 0.33 0.016 0.005 12.78 0.22 0.06 0.016 0.004 0.004 — — 170.038 0.21 0.28 0.012 0.004 9.56 0.18 0.05 0.016 0.005 0.004 — — 180.029 0.19 0.26 0.012 0.005 9.30 0.20 0.05 0.014 0.004 0.004 — — 190.036 0.24 0.25 0.014 0.004 9.16 0.20 0.40 0.009 0.005 0.005 1.05 — 200.019 0.31 0.30 0.015 0.004 8.54 0.19 0.05 0.020 0.004 0.004 0.31 1.7121 0.022 0.26 0.45 0.010 0.002 9.14 0.22 0.04 0.001 0.005 0.003 — — 220.034 0.19 0.29 0.015 0.005 8.96 0.24 0.03 0.026 0.004 0.004 — — 230.033 0.22 0.45 0.013 0.003 9.41 0.28 0.04 0.045 0.006 0.004 — — 240.028 0.24 0.41 0.014 0.002 9.23 0.12 0.06 0.006 0.004 0.003 — — 250.026 0.21 0.44 0.016 0.003 7.18 0.17 0.08 0.047 0.004 0.004 — 2.84 260.018 0.19 0.29 0.015 0.003 10.41 0.10 0.02 0.019 0.005 0.004 0.86 — 270.036 0.32 0.32 0.014 0.002 13.77 0.24 0.09 0.014 0.005 0.003 0.40 1.4828 0.026 0.17 0.36 0.014 0.001 7.18 0.05 0.06 0.015 0.006 0.004 — — 290.027 0.19 0.33 0.013 0.003 9.33 0.09 0.10 0.010 0.004 0.003 — — 300.018 0.21 0.29 0.012 0.002 9.41 0.20 0.05 0.003 0.004 0.005 — —Comparative Example 31 *0.062  0.23 0.25 0.013 0.003 9.11 0.16 0.040.018 0.005 0.004 — — 32 *0.088  0.20 0.26 0.014 0.003 9.23 0.14 0.050.020 0.004 0.004 0.95 0.05 33 *0.056  0.23 0.32 0.011 0.005 9.46 0.200.04 *0.053  0.006 0.004 — — 34 *0.074  0.28 0.28 0.014 0.004 10.50 0.190.05 *0.056  0.004 0.004 — —

[0068] TABLE 2 Density of Chemical Composition (mass %, bal.: Fe andimpurities) Precipitate Hot No. Ni Cu Co Ca Mg B Others NormalizingTempering (×10⁸ particles/mm²) Workability Steel of This Invention 1 — —— — — — — 1180° C. × 0.5 h 770° C. × 1 h 0.126 ◯ 2 — — — — — — — ″ ″0.302 ◯ 3 — — — — — — — ″ ″ 0.156 ◯ 4 — — — — — — — ″ ″ 0.070 ◯ 5 — — —— — — — ″ ″ 0.102 ◯ 6 — — — — — — — ″ ″ 0.148 ◯ 7 — — — — — — — ″ ″0.310 ◯ 8 — — — — — — — ″ 770° C. × 3 h 0.847 ◯ 9 — — — — — — — ″ —0.005 ◯ Comparative Example 10 — — — — — — — ″  770° C. × 10 h *2.069  ◯11 — — — — — — — ″  700° C. × 10 h *1.726  ◯ 12 — — — — — — —  900° C. ×0.5 h 770° C. × 1 h *1.426  ◯ 13 — — — — — — — 1180° C. × 0.5 h  700° C.× 10 h *1.968  ◯ Steel of This Invention 14 — — — — — — — ″ 770° C. × 1h 0.342 ◯ 15 — — — — — — — ″ ″ 0.241 ◯ 16 0.61 — — — — — — ″ ″ 0.165 ◯17 0.05 1.76 — — — — — ″ ″ 0.294 ◯ 18 — — 2.65 — — — — ″ ″ 0.198 ◯ 19 —— — — — — — ″ ″ 0.231 ◯ 20 0.43 1.51 — — — — — ″ ″ 0.145 ◯ 21 — — — — —— — ″ ″ 0.187 ◯ 22 — — — — — — — ″ ″ 0.201 ◯ 23 — — — — — — — ″ ″ 0.216◯ 24 — — — — — — — ″ ″ 0.205 ◯ 25 — — 2.45 — — 0.0010 Nd:0.026 ″ ″ 0.215◯ 26 0.96 — — 0.0018 — — — ″ ″ 0.179 ⊚ 27 — 1.88 — — — — — ″ ″ 0.325 ◯28 — — — — — — Ta:0.048 ″ ″ 0.166 ◯ 29 — — — — — 0.0032 ″ ″ ″ 0.181 ◯ 30— — — — 0.0023 — ″ ″ ″ 0.136 ⊚ Comparative Example 31 — — — — — — — ″ ″*1.106  ◯ 32 — — — — — — — ″ ″ *1.624  ◯ 33 — — — — — — — ″ ″ *1.216  ◯34 — — — — — — — ″ ″ *1.286  ◯

[0069] First, specimens for structure observation were taken from eachsteel plate in the above-mentioned process, and 10 fields of view wereobserved at a magnification of 5000 using a scanning electron microscope(SEM). The sizes and number of carbides mainly of the M₂₃C₆ type and MXtype carbonitrides were determined, and the density per mm² of theprecipitation of those carbides and carbonitrides, which were notsmaller than 0.3 μm in diameter (major axis) was determined. The resultsobtained are also shown in Table 2. Creep test specimens were also takenfrom each steel plate and subjected to creep testing.

[0070] Then, one side of each steel plate was subjected to edgepreparation at an angle of 30° with a root face thickness of 1 mm. Twoplates thus prepared were then butt-welded by the TIG method in themanner of multilayer welding, using a filler metal with the samecomposition as the corresponding steel plate, whereby a welded joint wasproduced for each steel plates. The welding heat input was 12-20 kJ/cmNeither preheating nor inter-pass temperature control was carried out.All the welded joints showed no defects after welding, namely no hightemperature cracks or low temperature cracks or other defects. Thefiller metals were prepared from the corresponding steel plates by hotworking and machining.

[0071] The welded joints produced were subjected to post-welding heattreatment by maintaining them at 740° C. for 0.5 hour. Then, creep testspecimens were taken from the welds and subjected to creep testing. Forsome welded joints (No. 1-9 and 14-30), V-notched specimens specified inJIS Z 2202 were taken from the welded joints and subjected to a Charpyimpact test. The creep test specimens were taken so that the weld linemight be located in the middle in the longitudinal direction. TheV-notched specimens were taken so that the melting boundary might belocated on the notch bottom.

[0072] The creep test was carried out at 650° C., and the data obtainedwere linearly extrapolated in order to determine the estimated strengthafter 3000 hours. The strength of each base metal was compared with thatof the welded joint, and the joint was evaluated as being satisfactorywhen the strength of the welded joint was 90% or more of that of thebase metal, and as being unsatisfactory when less than 90%.

[0073] The Charpy impact test was carried out at −20° C., and theadsorbed energy was determined. When the adsorbed energy was not lessthan 40 J, the specimen was evaluated as satisfactory.

[0074] The results, obtained in the above manner, are shown together inTable 3. TABLE 3 Ratio of Extimated Creep Strength Absorbed Strength(MPa) Welded Joint/ Energy No. Base Metal Welded Joint Base Metal (J) at−20° C. Steel of This Invention 1 75 70 0.93 64 2 78 72 0.92 62 3 74 690.93 66 4 71 67 0.95 52 5 73 69 0.94 62 6 73 68 0.93 62 7 78 72 0.92 658 76 68 0.90 67 9 78 78 1.00 65 Comparative Example 10 15 49 *0.65 — 1178 55 *0.70 — 12 76 55 *0.72 — 13 76 52 *0.68 — Steel of This Invention14 81 74 0.91 67 15 80 74 0.92 62 16 74 70 0.94 60 17 76 70 0.92 62 1875 70 0.93 64 19 75 69 0.92 64 20 79 73 0.93 62 21 74 69 0.93 64 22 7570 0.93 66 23 75 70 0.93 66 24 74 68 0.92 67 25 78 73 0.93 62 26 78 730.94 64 27 80 74 0.92 64 28 74 68 0.92 62 29 75 69 0.92 62 30 73 69 0.9464 Comparative Example 31 78 62 *0.80 — 32 79 51 *0.65 — 33 78 62 *0.79— 34 78 59 *0.75 —

[0075] As is apparent from Table 3, for each of the welded joints Nos.1-9 and 14-30 obtained by using the steel plates under the conditionsspecified by this invention, the estimated strength of the joint is notless than 90% of the estimated strength of the base metal. These weldedjoints had a sufficient level of toughness, with the absorbed energymeasured at −20° C., which is not less than 52 J.

[0076] On the contrary, for the welded joints Nos. 10-13 obtained byusing steel plates which showed a density of precipitates of carbidesmainly of the M₂₃C₆ type and MX type carbonitrides with a grain diameternot smaller than 0.3 μm, which were outside the range specified hereindue to inadequate heat treatment in steel plate production although therespective chemical compositions were within the range specified herein,the estimated strength of each joint was 65-72% of the strength of thecorresponding base metal; the HAZ softening was remarkable For weldedjoints Nos.31-34 obtained by using steel plates which showed a C and/orN content, and a density of precipitates of carbides mainly of the M₂₃C₆type and MX type carbonitrides, with a grain diameter not smaller than0.3 μm, both outside the ranges specified herein, the estimated strengthof each joint was 65-80% of the estimated strength of the correspondingbase metal and the HAZ softening was significant.

[0077] Industrial Applicability

[0078] The ferritic heat-resisting steels of the invention show a lowlevel of decrease in creep strength in the welding heat affected zoneTherefore, they are useful as materials for the construction of weldedstructures such as boilers

1. A ferritic heat-resisting steel that shows a low level of weldingheat affected zone softening, and is characterized by consisting of, bymass %, C: less than 0.05%, Si: not more than 1.0%, Mn: not more than2.0%, P: not more than 0.030%, S: not more than 0.015%, Cr: 7-14%, V:0.05-0.40%, Nb: 0.01-0.10%, N: not less than 0.001% but less than0.050%, sol. Al: not more than 0.010%, and O (oxygen): not more than0.010%, with the balance being Fe and impurities, and furthercharacterized in that the density of carbide and carbonitrideprecipitates contained therein with a grain diameter of not less than0.3 μm is not more than 1×10⁶/mm².
 2. A ferritic heat-resisting steelaccording to claim 1 that contains either or both of Mo and W, with atotal content of 0.1-5.0 mass % in lieu of part of Fe.
 3. Aferriticheat-resisting steel according to claim 1 or 2 that contains one or moreof Cu, Ni and Co, with a total content of 0.02-5.00 mass % in lieu ofpart of Fe.
 4. A ferritic heat-resisting steel according to any ofclaims 1 to 3 that contains one or more of Ta, Hf, Nd and Ti, with atotal content of 0.01-0.20 mass % in lieu of part of Fe.
 5. A ferriticheat-resisting steel according to any of claims 1 to 4 that containseither or both of Ca and Mg, with a total content of 0.0005-0.0100 mass% in lieu of part of Fe.
 6. A ferritic heat-resisting steel according toany of claims 1 to 5 that contains 0.0005-0.0100 mass % of B in lieu ofpart of Fe.